(1) Field of the Invention
The present invention relates to single-photon generating device, single-photon detecting device (photodetector) and optical quantum gate acting as key devices for quantum information processing technology, particularly, quantum cryptography communication.
(2) Description of Related Art
In the BB84 protocol, a main stream protocol for quantum cryptography communication at present, cryptographic communication preventing wiretapping is realized by transmitting quantum information carried by a single photon.
For this purpose, a single-photon generating device for securely generating a photon one by one in a single pulse is necessary as a signal source (light source). However, it is difficult to realize such a single-photon generating device.
Accordingly, up to the present, a single-photon generating device has been realized in a simulated manner by weakening laser light so that the mean number of photons per pulse becomes on the order of 0.1 particle.
However, since the laser light is coherent light, there is a case that two photons or more are generated even when using such the weakened light. Thus, information may possibly be leaked to a wiretapper, of which probability is not negligible. Also, there is a problem that communication speed is extremely decreased when intending to carry out long distance communication safely. Therefore, in order to realize high-speed quantum cryptography communication, it is inevitable to realize a single-photon generating device.
As methods for realizing such the single-photon generating device, a variety of methods have been proposed so far.
Among such methods, as a promising technique for achieving high reliability in a telecommunication band, there is a single-photon generating device using light emission from a localized level in a solid substrate, particularly from an exciton level in a low-dimensional semiconductor represented by a quantum dot.
However, since the dielectric constant of a semiconductor is as large as three or more, the light generated from the quantum dot located inside the substrate is almost reflected on a substrate surface, and the ratio of the light outputting from the substrate surface to air is a few percent. Among the above output light, the light coupled to an optical fiber using, for example, an objective lens is merely 1% or less. Therefore, it is an important issue to increase extraction efficiency.
Accordingly, in order to increase the extraction efficiency in the single-photon generating device using light emission from the quantum dot in the semiconductor substrate, a device having the following structure has been proposed.
Namely, there has been proposed a method of forming a single-photon generating device including a self-organized quantum dot layer of InAs, and a DBR (Distributed-Bragg reflector) microresonator (microcavity, or simply, cavity) constituted of a DBR mirror of GaAs and AlAs through the epitaxial growth, and thereafter through etching onto a minute cylinder (micropost), so as to output substantially entire light inside the micropost cavity from the upper (surface) side (For example, refer to “Efficient Source of Single Photons: A Single Quantum Dot in a Micropost Microcabity”, Matthew Pelton et al., Physical Review Letters, Volume 89, Number 23, Dec. 2, 2002.).
In the above structure, the coupling with a particular confinement mode can be strengthened in the cavity, thanks to the Purcell effect. It is possible to restrain light dissipation to an unintended direction, and to extract the light to a particular direction only. Thus, the extraction efficiency can be improved. Also, with a shortened emission lifetime thanks to the Purcell effect, it becomes possible to obtain such effects as an increase of a photon generating rate, as well as the mitigation of an influence of decoherence.
Further, by providing an electrode on the semiconductor layer including the quantum dot, additional functions may be provided in the single-photon generating device, such as enabling light emission through current injection, and varying a light emission wavelength by applying an electric field. In such an EL single-photon generating device, and an electric-field-controlled, variable-wavelength PL single-photon generating device, there are provided conductive semiconductor layers of, for example, GaAs on the upper and lower sides of a quantum dot layer of, for example, InAs, and also contact electrodes on the above conductive semiconductor layers (For example, refer to “Electrically Driven Single-Photon Source”, Zhiliang Yuan et al., Science, Vol. 295, Jan. 4, 2002.).
However, according to the structure such as proposed in the above paper “Efficient Source of Single Photons: A Single Quantum Dot in a Micropost Microcabity”, there is a problem that, when a photon ejects from the internal cavity to the external space, the space distribution thereof expands to a considerable extent, and as a result, the efficiency at the time of coupling to an optical fiber using, for example, an objective lens decreases to a non-negligible extent.
Also, because it is necessary to form a considerably thick hetero epitaxial growth layer for a DBR mirror on both upper and lower sides of the quantum dot layer, quality of the quantum dot may possibly be affected.
Further, because it is necessary to accurately manufacture an extremely long pillar shape including the DBR mirror, an advanced etching technique is required.
In particular, since the quantum dot (formed of, for example, InP) for emitting light of a telecommunication band (for example, 1.5-μm band) has a high strain stress internally, it is highly possible that dry etching may cause quality degradation of the quantum dot.
Further, in the EL single-photon generating device and the electric-field-controlled, variable-wavelength PL single-photon generating device, desirably, at least one of the electrodes is disposed in the vicinity of the quantum dot layer so as to efficiently perform current injection and electric field application to the quantum dot layer.
Accordingly, in the device described in the above paper “Electrically Driven Single-Photon Source”, in order that only the light generated from one quantum dot (single photon) is output to the outside, a hole is produced in the contact electrode (metal electrode) provided on the upper portion of a mesa structure, and the light is extracted from the above hole (that is, from the surface side of the mesa structure).
However, it is difficult to increase the extraction efficiency with such the structure.